Conventionally, in order to cut a silicon ingot, a multi-wire saw is used, which is capable of cutting with a small cutting allowance and a uniform thickness, and capable of cutting a number of wafers at a time. Such cutting of a silicon ingot using a multi-wire saw is performed by introducing a slurry containing abrasive grains into a cutting interface while pressing a silicon ingot against a traveling wire (for example, see Patent Document 1).
In the cutting of a silicon ingot using a wire, the temperature of the slurry increases along with the proceeding of the cutting of an ingot with the wire, and a roller of the wire saw expands/contracts. The machining conditions for the ingot vary, which makes it impossible to maintain the machining precision of an ingot constant. Therefore, an attempt is made to ensure the machining precision by cooling the slurry (for example, see Patent Document 2).
However, recently, there is a demand with respect to the cutting of a silicon ingot, for reducing a wafer machining cost by decreasing a cutting allowance and a cutting pitch while maintaining the high quality of a wafer. In order to decrease the cutting allowance, the diameter of a wire may be decreased. In this case, the breakage strength of the wire also decreases accordingly, which makes it necessary to decrease the tension applied to the wire. An ingot is cut by a lapping action that is a pressure transfer. Therefore, when the tension of the wire is decreased, the cutting speed becomes low, and the displacement (deformation) of the wire increases. When the displacement (deformation) of the wire increases, the displacement of the wire in a direction orthogonal to the cutting direction also increases, with the result that the warpage, irregular thickness, and minute unevenness (saw marks) occur, decreasing the quality of the wafer. When the feeding speed of a silicon ingot is decreased in accordance with the delay in the cutting speed in order to decrease such warpage of the wire, the cutting efficiency decreases. When the feeding speed of a silicon ingot is increased by increasing the feeding speed of a wire so as to compensate for the delay in the cutting speed, a margin with respect to a dispersion defect of abrasive grains at the cutting interface is lost, and the wire breaks owing to the abrupt increase in tension. Thus, in order to maintain the high quality of a wafer, and decrease the cutting allowance and cutting pitch of a silicon ingot, it is necessary to reduce the cutting resistance.
A method of cutting a silicon ingot using a fixed abrasive grain wire and a slurry containing free abrasive grains or a KOH alkaline solution in concentration of 2% or less has been proposed (for example, see Patent Document 3).
On the other hand, in the multi-wire saw, when the slurry is supplied to the wire, a large amount of slurry in a curtain shape is discharged from above the wire. A very small amount of the supplied slurry adheres to the wire, and most of the slurry drops below the wire. Therefore, the dropped slurry is collected by a slurry storage tank via a drain pulled out of a machining chamber floor portion. More specifically, most of the slurry is not used for cutting a silicon ingot, and is merely circulated to the slurry storage tank via a slurry supply path and a discharge mechanism from the slurry storage tank. Generally, in the multi-wire saw, the machining chamber is kept in a negative pressure state so as to discharge hydrogen generated by reaction. Therefore, water contained in the slurry is evaporated in the course of supply and circulation of the slurry. In particular, when the temperature of the slurry to be used is high, water is evaporated remarkably, the viscosity of the slurry supplied and circulated becomes high, and the excess slurry that is not introduced to the cutting interface is dried and solidified in an inlet portion through which the wire is inserted in the silicon ingot. The change in the viscosity of the slurry causes a decrease in machining quality (variation in wafer thickness, generation of saw marks). Furthermore, when the solidified slurry is accumulated in the inlet portion through which the wire is inserted in the silicon ingot, the resistance is generated between the wire and the slurry, or chips of the solidified slurry are interposed at the cutting interface between the wire and the silicon ingot, which may eventually lead to the breakage of the wire. When the wire breaks during machining, not only the machining is interrupted but also the silicon ingot that is being machined is wasted. In order to resume the machining, man-hours are required for a collection operation of a silicon ingot, a cleaning operation, a wire stretching operation, and the like, resulting in a significant decrease in productivity.
Patent Document 1:JP 3187296 B
Patent Document 2:JP 08-47850 A
Patent Document 3:JP 2000-343525 A